1. Field of the Invention
The present invention relates to the adjustment of a white balance of a CRT display system (television (TV) receiver, monitor, etc.). In particular, it relates to a method and apparatus for adjusting the white balance of a CRT display system which enables speedy simultaneous adjustment of the background and the level, of extremely high resolution (definition, density) monitors such as graphic display monitors operating at a high frequency over 100 MHz as the band of the video signal.
2. Description of the Related Art
In a color CRT display system, when monochrome image data is reproduced, the same color temperature must be ensured despite the various brightnesses of the screen. To obtain such an image, the white balance is adjusted, i.e., the drive voltage of the video signal amplifier inside the color CRT display system is adjusted.
To adjust a white balance, the light energies emitted from the three red, green, and blue color phosphors must be adjusted to a predetermined ratio. The light efficiencies of the light emitted from the currently used three color phosphors differ slightly, but the ratio of currents of the red, green, and blue electron beams for obtaining a white color is substantially the same. Therefore, it is sufficient to slightly adjust the ratio of the currents. There is some variation, however, with individual color CRT systems, so a circuit is provided for the adjustment of the white balance.
Usually, the white balance is adjusted by two types of adjustment: adjustment of the background to adjust the black levels of the individual color signals and adjustment of the drive voltage to adjust the white peak level of the video signal so that the pedestal level of the video signals (level close to substantially black) gives a completely black display.
In a color CRT display system, a video signal output amplifier provided at the last stage and the cathode electrode terminal of a high resolution monitor CRT unit in the CRT display system are connected through a coupling capacitor. In a synchronous-signal-tip level clamping (STC) video circuit, the ratio between the pedestal level of the video signal and the tip level of the synchronous signal is usually set to 0.714:0.286.
When adjusting the white balance of a CRT display system before factory shipment, if the pedestal level is adjusted for the drive Voltage, the tip level of the synchronous signal will be adjusted in addition to the level of the video signal. That is, the cathode of the CRT display system has been clamped at a tip level of the synchronous signal, so due to the adjustment of the drive voltage, the pedestal level showing the black level and adjusted by the adjustment of the background may be changed against the will of the adjuster. As a result, adjustment of the background is required once again after the adjustment of the drive voltage. It then becomes necessary to adjust the drive voltage after this adjustment of the background. This means that one must repeatedly adjust the white balance by going back and forth between adjustment of the background and adjustment of the drive voltage, and, consequently, the adjustment takes time.